Equol, a non-steroidal estrogen of the isoflavone class, is the most important metabolite of ingested soy isoflavones. It is made by intestinal bacteria and not found in the urine and blood of infants before 4-months of age. For unknown reasons only one-third of adults consuming soy foods make equol. Recent studies of osteoporosis prevention, cardiovascular health, and menopause have shown that beneficial effects from soy foods are significantly greater in people who are 'equol-producers' compared with those unable to make equol, and it is now realized that these two distinct populations need defining in dietary intervention studies. While equol exists in two enantiomeric forms, we have shown that humans make exclusively S-equol, and it has a high affinity for estrogen receptor ER-beta and shows negligible binding to ER-alpha. R-equol on the other-hand has potent antiandrogen properties, antagonizing the actions of dihydrotestosterone, making it of pharmacological interest. For the first time, the pharmacokinetics of S- and R-equol will be determined. Our aim is to prove that S-equol occurs in human plasma and urine not because of differences in the absorption of the two enantiomers, but moreover due to its bacterial enantiomeric-specific formation. Since there are advantages to being an equol-producer it is important to understand the factors governing equol production. We will determine when equol first appears in early life and whether it is differences in the type of early infant nutrition, or the composition of the post-weaning diet that predispose to the production of equol. Breastfeeding leads to differences in intestinal bacterial colonization and a lower pH compared with bottle-feeding, and this is expected to facilitate equol formation. Preliminary in vitro and human data suggest higher intakes of certain prebiotic macronutrients conducive to colonic fermentation favor equol formation. This will be determined by comparing equol-production in healthy adults relative to their dietary intakes of macronutrients using fiber intake determined from food frequency questionnaire and 3-day diet records to stratify groups. We will determine the long-term stability of equol-production in adults and its response to antibiotic use. Given the clinical relevance of equol, a greater understanding of factors governing its production will facilitate future strategies to manipulate equol production and enhance the overall clinical effectiveness of soy foods.